Discharge tube having its electrodes recessed in wells



June 6, 1967 J. F. WAYMOUTH E AL 3,324,332

DISCHARGHTUBE HAVING ITS ELECTRODES BECESSED lN WELLS Filed Oct. 24, 1966 I i 5 l2 19 FREDERIC KOURY JOHN F. WAYMOUTH INV NTORS B (AL M ATTOEZZI E'Y United States Patent DISCHARGE TUBE HAVING ITS ELECTRODES RECESSED IN WELLS John F. Waymouth, Marblehead, and Frederic Koury,

Lexington, Mass., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Oct. 24, 1966, Ser. No. 589,073 Claims. (Cl. 313-220) This application is a continuation in part of our copending application, Ser. No. 313,101, filed Oct. 1, 1963, now. abandoned. This invention relates to high pressure electric discharge devices having fills including halogens and particularly to a novel are tube construction which delivers maximum light emission from such devices.

High pressure electric discharge devices containing halogens have been disclosed in the art, however, the arc tubes which were used previously have been those which were designed for fills of mercury alone. But we have discovered that such fills require an entirely new are tube construction with substantially different geometry to attain maximum light emission.

Accordingly, the primary object of our invention is increasing the light emission of high pressure electric discharge devices containing fills including halogens.

A feature of our invention is precise positioning of the electrode within the arc tube and a particular disposition of the electrode near the glass seal.

Another feature of our invention is disposing the electrodes in Wells at either end of the arc tube, the diameter of the Wells bearing a particular relationship to the diameter of the electrodes.

Another feature of our invention is the discovery that the width of the seal should bear a particular relationship to the diameter of a portion of the arc tube.

The many other objects, features and advantages of the instant invention will become manifest to those conversant with the art upon reading the following specification when taken in conjunction with the accompanying drawings wherein preferred embodiments of the arc tube construction of our invention are shown and described by way of illustrative examples.

Of these drawings:

FIGURE 1 is an elevational view of the high pressure electric discharge device according to our invention.

FIGURE 2 is a fragmentary, enlarged, cross sectional view of the end of our high pressure electric discharge device.

Referring to FIGURE 1 of the drawing, an arc tube is shown which finds applicability in lamps such as described in the applications of Waymouth et al., Ser. No. 191,162, filed Apr. 30, 1962, now abandoned, and entitled, High Pressure Electric Discharge Device; Waymouth et al., Ser. No. 209,974, filed July 16, 1962, and entitled, High Pressure Discharge Device; Waymouth et al., Ser. No. 230,944, filed Oct. 16, 1962, and entitled, Electric Discharge Device, and Butler et al., Ser. No. 239,272, filed Nov. 21, 1962, now abandoned, and entitled, Electric Discharge Device.

Sealed at opposite ends of the arc tube, in press seals 1 and 2 are electrodes which comprise wire helixes 3 and 4 mounted upon rods 5 and 6 respectively. The rods 5 and 6 of the electrodes can be fabricated of molybdenum or tungsten wire 0.008 to 0.075 inch in diameter. Generally, the wire helixes are made of tungsten, also being 0.008 to 0.075 inch in diameter. When wrapped upon the rods 5 and 6, the helixes have an outside diameter of 0.024 to 0.225 inch. Although cathodes having a sliver of thorium disposed between the helix and the rod are generally preferred, we can use thoriated tungsten wire or in some cases utilize thorium added directly to the arc tube. When the spectrum of a different metal is desired, thorium can ice be eliminated. An auxiliary probe or electrode 7, generally of tantalum or tungsten, is provided at one end of the arc tube adjacent a main electrode. When desired to facilitate starting, a heater coil such as described in the application of Waymouth, Ser. No. 302,656, filed Aug. 16, 1963 may be added. The are tube is formed of glass conventionally used by the art such as quartz, Vycor (an almost pure silica glass) or the alumina types.

Each of cores 5 and 6 of the electrodes and the starting probes 7 are sealed in the press seals 1 and 2 and electrically connected to lead-in wires 8, 9 and 10 through molybdenum foil sections 11. As is conventional in the art, the various lead-in wires are spot welded to the foil sections although other means of attachment are possible as long as sound electrical connections are made. The foil sections are very thin, for example approximately 0.0008 inch, and go into tension when the glass sealed about them cools.

The ends of the arc tube are sealed by press seals 1 and 2 which extend across the entire width of the respective ends of the wells 12 and 14. Sealed within the press seals 1 and 2 are the rods 5 and 6, respectively. An inwardly diverted curvilinear section 20 is disposed between each end of the cylindrical arc tube and the mouths of each of the wells 12 and 14. A press seal section 21 extends from the well 14.

We have discovered that the halogen-containing arc tubes require certain relationships between the press seal width and the well width to attain maximum light emission and red color rendition. If the press seal width is too great, the arc formed within the arc tube will be cooled and all of the fill will not be vaporized into the arc stream. Of course, the width of the press seals 1 and 2 can be no less than the diameter of the wells 12 and 14. Particularly, we have discovered that the ratio of the width of the press seals to the internal diameter of the wells must be no greater than about two. As shown in Table I following, ratios greater than about two reduce the brightness and the red color rendition. Each of the lamps tested operated at 400 watts and were of the same construction and fill except for the indicated variations of the seal width.

TABLE I Interior Ratio Seal Test Seal Well Width to Lumens Percent Width, Diameter, Well Red mm. mm. Diameter In addition to the ratio of seal width to well width, certain other construction features are also found to be quite important. The distance which the rearward ends of the tungsten helixes 3 and 4. extend from the base of the respective wells 12. and 14 appreciably affects the light emission and the red color rendition. If the rear ends of the helixes extend more than 1.5 mm. beyond the bases of the respective wells, the condensible materials will tend to deposit therein and will not enter the arc stream. If the rear ends of the helixes are closer than 0.5 mm. to the bases of the wells, the quartz will become so hot that it will devitrify and ruin the arc tube. In Table II following, the tests on a series of 400 watt lamps illustrate the necessity of maintaining the precise distance indicated above. In all respects, apart from the distance, the lamps were substantially identical.

1 Lamp rendered inoperative.

Other important features of our invention are the use of a well at either end of the arc tube connected thereto by curvilinear portions. Furthermore, we have discovered that the distance which the wire helix portions of the electrodes extend from the mouths of the wells is to be controlled. If the helixes are disposed too deeply within the well, the light emission of the lamp will be substantially reduced, as set forth in Table III. In each arc tube test-ed, similar fills were used and the distance between the rear of the helix and the base of the well was constant. A 55 mm. arc length was used.

Hence it is apparent from the foregoing table, that unless the distal ends of the helixes are at least flush with the mouths of the wells, the brightness is reduced. As the ends of the coils are disposed further out from the wells, the light output is again reduced, however the reduction is not as great as when the helixes are recessed. Hence, the distal ends of each of the electrodes are preferably disposed so that they are at least ifiush with the mouth of. the respective wells but no more than one half to three quarters of the helixes extending outwardly therefrom. Moreover, if the diameter of the well is reduced, then the operating temperature can be made sufficiently high to produce migration of the fill metals into the arc stream. In that case, the helix can be wholly received within the well. In Table III, the brightness is slightly lower than in the other tables because the fill has been modified.

In order to allow for even heat distribution around the helix and upon the wall of the well, we have found that the diameter of the well section 21 should be no more than about four times the diameter of the helix 3, diameter of the well being measured at its mouth. Such construction compensates for the problems involved in attempting to attain precise positioning upon the axis of the arc tube. Hence, even through a coaxial relation between the helix and the well is not precisely attained in all are tubes when manufacturing, no portion of the helix will be so close to the well wall to heat it excessively. Excessive heat would cause devitrification of the quartz and ruin the arc tube. When a diameter ratio greater than about 3 to 4 is used, the condensible materials will deposit upon the walls and not pass into the arc stream. However the upper limit may be modified in some cases and relatively small diameter electrode may be used if means are incorporated in the arc tube for increasing the temperature of the Wall. For example, it has been common practice to paint a reflective gold surface on the exterior of the well walls to reflect the heat back onto the helix 3 and help vaporize condensibles.

In FIGURE 2, an enlarged view of one end of the arc tube is shown. The well 14 is disposed between the generally conically shaped curvilinear portion 16 and the press seal 2.. Curvilinear portion 16 is part of the arc tube and is the connection between cylinder portion 18 and well 14.

We have found that the most desirable press seal is that which radiates the least amount of heat from its surface. A most advantageous construction is one which extends straight down from the well and has thickened edges 15. Because the edges are thickened, the arc tube will be more durable and less prone to breakage.

Extending through the seal and adjacent to the electrode 3 is the starting probe 7 which preferably enters the arc tube at a point offset from the axis and is bent over to come within 1 to 3 mm. from the end of the electrode.

In general, the halogen and light emitting metal containing arc tubes are used for the production of white light, continuous spectrum or even monochromatic light.

When using the halogens, a ratio of iodine atoms to mercury atoms should be maintained at approximately 0.45 for maximum efiiciency. We have found that we can use ratios greater or less than 0.45 (within definite limits) but the emission is reduced using such off peak ratios. Since it is difficult, if not impossible, to produce lamps in production lines wherein the ratio of mercury to iodine is exactly 0.45, tolerances are allowed between 0.01 to 0.85 and within such tolerances, reasonable efilciency is still evidenced. When a white light is desired we must include thorium or another suitable metal as a continuum-emitting materials. Particularly describing a thorium-type lamp, the arc may be operated either with or without the conventional 5 mg. thorium sliver disposed inside the helix or each electrode. When using thorium slivers, 2.6 l0 to 2.6 10- gram atoms of thorium per centimeter of arc length (excluding the slivers) must be included and when no thorium slivers are used 5.2 10' to 52 10- gm. atoms of thorium per centimeter of arc length should be added. In the former case, it is preferable to add 1.6 10 gram atoms of thorium per centimeter of arc length. The thorium may be added either as the metal per se, or as the anhydrous iodide. Above this range, the thorum metal tends to deposit upon the walls of the arc tube while below the range a continuous spectrum is evidenced. As we have explained in the copending application concerning thorium, indicated previously, by continuous spectrum we mean an almost complete forest of spectral lines spaced generally less than 5 A. apart and containing superimposed on the forest, the typical mercury spectrum with lines at 4048, 4348, 5461, 5770 and 5790 A. With other metal additives, the continuous spectrum can be a true band spectrum with no lines being apparent except the mercury lines.

Additionally, it is often desirable to add 5.2 l0- to 1.6 10* gram atoms of sodium per centimeter of arc length. Without the sodium the voltage drop is often too high to operate the lamp on conventional ballasts. But when more than the stated upper limit is used, the excess sodium condenses upon the wall. The sodium may be added either as the metal or the corresponding iodide.

The sealing techniques and positioning of the electrodes in the high pressure electric discharge device according to our invention takes place in a manner quite similar to that known to the art with conventional mercury lamps. And further, the mercury metal may be added to the arc tube by techniques well known to the art. To prepare the arc tube, we pump down an envelope having a pair of electrodes disposed at either end thereof, through an exhaust tubulation expending from the surface of the envelope and disposed in communication with the interior thereof. The envelope is then baked to elevated temperatures of 800 C. and filled With argon to flush out residual impurities. It is quite important to eliminate or substantially eliminate the presence of hydrogen from the arc tube. Hydrogen is known to effect adversely the starting of mercury lamps but its effect appears to be greater in the lamps prepared according to our invention. The difficulty when hydrogen is inad vertently included is that hydrogen iodide tends to form. This compound has a much higher vapor pressure than any other iodide present. We believe that for every atom of hydrogen, that there is an extra atom of iodide in the vapor state. Presence of the iodine in the vapor state increases the voltage which must be applied to the lamps for starting. Hence, not only must hydrogen be substantially eliminated from the gases in the filling of the tube but each and every part going into making up the arc tube must be free of residual hydrogen impurities. For example, the electrodes can be vacuum baked at 600 to 860 C. for a few hours before their use to eliminate hydrogen which might occur due to processing. Furthermore, care should be exercised when sealing the electrodes into the arc tube to prevent hydrogen-containing, combustion gases from seeping in or becoming absorbed upon the surface.

The pump and fill procedure above described is usually repeated three to four times and then an arc is struck between the electrodes while there is a filling of argon gas. This operation of the arc removes any residual impurities from the electrodes and these contaminants can then be easily drawn from the system when the argon filling is pumped out. We can then add approximately 34 milligrams of mercury, 13 milligrams of mercuric iodide and 5.0 milligrams of thorium to an envelope having an arc length of approximately 8.3 centimeters. The are tube is then filled to atmospheric pressure with argon gas which is slowly leaked out until a pressure of about 23 milligrams of mercury is obtained. Subsequently, the exhaust tubulation is tipped off and the envelope is sealed. Testing of the lamp indicates that a white light emission is evidenced, which emission is in the order of 75 lumens per watt.

It is apparent that modifications and changes may be made within the scope of the instant invention but it is our intention, however, to be limited only by the scope of the appended claims.

As our invention, we claim:

1. A high pressure electric discharge device containing a fill of halogen atoms, mercury atoms and atoms of a continuum forming metal, said device comprising: a generally cylindrical arc tube formed of a high melting glass; a well of a diameter reduced from the diameter of said are tube diposed at each end thereof; an inwardly directed, curvilinear section disposed between each end of the cylindrical arc tube and the mouths of each of the wells; a press seal disposed at the ends of each of said wells; a helix mounted upon a rod disposed in each of said wells, said rods extending into said press seals, the inwardly extending, distal end of each of the helixes being disposed within the respective well; the width of each of said press seals being no more than about twice the internal diameter of the respective wells.

2. The device according to claim 1 wherein the rear ends of each of the helixes are spaced from the base of the respective wells by a distance between about 0.5 to 1.5 mm.

3. The device according to claim 1 wherein the diameter of each well is no more than about four times the outside diameter of the helixes.

4. The device according to claim 1 wherein the press seals have thickened edges.

5. A high pressure electric discharge device containing a fi'll of halogen atoms, mercury atoms and atoms of a continuum forming metal, said device comprising: a generally cylindrical arc tube formed of a high melting glass; a well of a diameter reduced from the diameter of said are tu be disposed at each end thereof; an inwardly directed, curvilinear section disposed between each end of the cylindrical arc tube and the mouths of each of the wells; a press seal disposed at the ends of each of said wells; a helix mounted upon a rod disposed in each of said wells, each of said rods extending into each of said press seals, the inwardly extending, distal end of each of the helices being disposed within the respective Well; the diameter of each well being no more than about four times the outside diameter of the helix; the width of each of said press seals being no more than about twice the internal diameter of the respective wells.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

S. SCHLOSSER, Assistant Examiner. 

1. A HIGH PRESSURE ELECTRIC DEVICE CONTAINING A FILL OF HALOGEN ATOMS, MERCURY ATOMS AND ATOMS OF A CONTINUUM FORMING METAL, SAID DEVICE COMPRISING: A GENERALLY CYLINDRICAL ARC TUBE FORMED OF A HIGH MELTING GLASS; A WELL OF A DIAMETER REDUCED FROM THE DIAMETER OF SAID ARC TUBE DISPOSED AT EACH END THEREOF; AN INWARDLY DIRECTED, CURVILIEAR SECTION DISPOSED BETWEEN EACH END OF THE CYLINDRICAL ARC TUBE AND THE MONTHS OF EACH OF THE WELLS; A PRESS SEAL DISPOSED AT THE ENDS OF EACH OF SAID WELLS; A HELIX MOUNTED UPON A ROD DISPOSED IN EACH OF SAID WELLS, SAID RODS EXTENDING INTO SAID PRESS SEALS, THE INWARDLY EXTENDING, DISTAL END OF EACH OF THE HELIXES BEING DISPOSED WITHIN THE RESPECTIVE WELL; THE WIDTH OF EACH OF SAID PRESS SEALS BEING NO MORE THAN ABOUT TWICE THE INTERNAL DIAMETER OF THE RESPECTIVE WELLS. 